explain that keys must be lower cased (#31586)

examples/python/auth/customized_auth_client.py

@@ -48,6 +48,7 @@ class AuthGateway(grpc.AuthMetadataPlugin):
         #     service_url=u'https://localhost:50051/helloworld.Greeter',
         #     method_name=u'SayHello')
         signature = context.method_name[::-1]
+        # NOTE: The metadata keys provided to the callback must be lower-cased.
         callback(((_SIGNATURE_HEADER_KEY, signature),), None)